As a power amplifier for amplifying linearly modulated signals and a low noise amplifier for use in a receiver unit of linearly modulated signals, an amplifier having high linearity is required to suppress deterioration of a transmission characteristic caused by a spectral characteristic and signal distortion.
In particular, when a multilevel amplitude modulation scheme is applied to radio communication, a technique is required on the transmission side for linealizing the amplification characteristic of the power amplifier, thereby suppressing nonlinear distortion and reducing adjacent channel leakage power. Further, while high power efficiency is generally required in the amplifier at all times, the linearity and the efficiency in the amplifier are generally contradictory characteristics. When it is intended to improve the power efficiency by use of an amplifier having a degraded linearity, a technique for compensating the distortion produced therefrom is essential.
As one system for compensating distortion, a predistortion system is known. The principle of the predistortion system is that a characteristic inverse to the distortion characteristic of the amplifier is added in advance to the input signals of the amplifier, so as to obtain desired non-distorted signals in the output of the amplifier. As to the predistortion system, detailed descriptions have been given in the following Patent documents 1, 2 and 3, for example.
A transmission signal before distortion compensation is compared with a demodulated feedback signal, and using the error thereof, a distortion compensation coefficient is calculated and updated. The distortion compensation coefficient is stored into a memory, using transmission signal amplitude, power, or the function thereof, as an address. Then, on the transmission signal to be transmitted next, predistortion processing is performed and output, using an updated distortion compensation coefficient. By repeating the above operation, finally, convergence is made to an optimal distortion compensation coefficient, and thus, the distortion in the transmission power amplifier is compensated.
[Patent document 1] The official gazette of the Japanese Unexamined Patent Publication No. Hei-9-69733.
[Patent document 2] The official gazette of the Japanese Unexamined Patent Publication No. 2001-189685.
[Patent document 3] The official gazette of the Japanese Unexamined Patent Publication No. 2000-278190.